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Re: [PATCH] x86/shadow: sh_type_to_size[] needs L2H entry when HVM+PV32
- To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 23 Jan 2023 11:47:03 +0100
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- Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, "Tim (Xen.org)" <tim@xxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 23 Jan 2023 10:47:20 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 23.01.2023 11:43, Andrew Cooper wrote:
> On 23/01/2023 8:12 am, Jan Beulich wrote:
>> While the table is used only when HVM=y, the table entry of course needs
>> to be properly populated when also PV32=y. Fully removing the table
>> entry we therefore wrong.
>>
>> Fixes: 1894049fa283 ("x86/shadow: L2H shadow type is PV32-only")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> Erm, why?
>
> The safety justification for the original patch was that this is HVM
> only code. And it really is HVM only code - it's genuinely compiled out
> for !HVM builds.
Right, and we have logic taking care of the !HVM case. But that same
logic uses this "HVM-only" table when HVM=y also for all PV types. Hence
the PV32-special type needs to have a non-zero entry when, besides HVM=y,
PV32=y as well.
> So if putting this entry back in fixes the regression OSSTest
> identified, then either SH_type_l2h_64_shadow isn't PV32-only, or we
> have PV guests entering HVM-only logic. Either way, the precondition
> for correctness of the original patch is violated, and it needs
> reverting on those grounds alone.
I disagree - the table isn't needed when !HVM, and as such can be
considered HVM-only. It merely needs to deal with all cases correctly
when HVM=y.
Jan
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